Battery-operated computer systems such as notebook personal computers, portable navigation devices, portable digital media players, and smart phones, have a need for technology that can automatically predict the remaining run time of the device, based on the current state of charge of its battery. To meet such a need, integrated circuit (IC) developers and manufacturers offer a battery fuel gauge circuit, also referred to as a gas gauge circuit. The basic gas gauge circuit may come in the form of a chip or IC package that is to be integrated in a host device, to be connected to the terminals of a battery pack of the device. The IC chip or package may be one that is integrated into the battery pack or is on a battery connector board, or it may be located directly on a main logic board of the device.
The gas gauge circuit contains various analog and digital circuitry needed to accurately measure battery voltage and/or battery current on an on-going basis (e.g., repeating such measurements every given time interval, as the device goes through its typical active and sleep usage cycles). This function is also referred to as a coulomb counter. The digital circuitry may include a microcontroller, i.e. a processor coupled to memory that stores instructions or software code to be executed by the processor. The microcontroller executes its programming, to measure or compute various parameters associated with usage of the battery as part of the device, such as cell voltage, average pack voltage, pack current, capacity change, battery impedance, open-circuit voltage, and others. It reports such information to a host controller, such as an application processor or power management unit of the host device, through a low overhead bus or interface such as a single wire HDQ serial data interface, a Smart Battery Specification, SBS, interface, or an I2C bus.
Some gas gauge circuits have a watchdog timer integrated in their designs. A watchdog timer monitors a specific periodic signal internal to the gas gauge and looks for its absence. Software running in the microcontroller of the gas gauge circuit is responsible for repeatedly acting (e.g., setting a particular bit of a register) to maintain this periodical signal. When the periodic signal stops, it is assumed that the gas gauge has “hung up” or entered a failure mode software loop. Thus, if the internal timer does not see a signal transition in a set amount of time, it will trigger a reset of the gas gauge (including its microcontroller).
In addition to the watch dog timer, the gas gauge circuit may also have power-on reset circuitry, which automatically resets the gas gauge when the supply voltage provided on its power supply pin cycles, by dropping to a sufficiently low level and then rising to a normal level. The supply voltage may drop to zero, for example when the battery pack has failed or has been removed from the device.